King Shantanu and his Son Devavridhan (Bheeshma)

King Shantanu was having a walk on thr river bank of Ganga when he felt the fragrance which he did not smell or feel till that time. Then he saw a very beautiful girl and knew that the smell was from her. He felt love stricken and immediately approached her, introduced himself  and begged her to marry him.
That girl agreed to marry him on some conditions. Shanthanu should not ask about her whereabouts, should not ask her name and question her actions even if it was so cruel and abhorrent. If at any time he questioned her action she would immediately go away, leaving him. Shantanu readily agreed to her conditions and she agreed to marry him and they got married immediately.
They had a very pleasant and happy life. She became pregnant and delivered a male child but immediately after birth she took the child to the river Ganga and immersed it into the water killing it instantly. She did this seven times and Shantanu just watched these in horror and since he promised not to question her action he kept quiet.
When the eighth child was born as usual the girl took the child to the river and was about to kill it then Shantanu  stopped her and asked her why she was doing such a cruel and intolerable act? She smiled and said "Oh king, you have broken your promise. So now I would leave you for ever. I am river Ganga. I became your wife on the request of ASHTA VASUS ( eight Vasus. They are Devas) . Once the Ashta vasus were playing near the hut of Sage Vashishta and saw the Holy Cow of Nandhini which was so beautiful and could give anything one wishes. One of the vasus wife wanted the cow and requested her husband to get her the cow. So the Ashta vasus took the cow along with them to heaven. When Vashishta came back he was furious and cursed the Ashta Vasus to become humans and live in the earth and suffer.

The Ashta Vasus came of the curse and came running back to sage Vashishta and begged him to pardon them. Sage Vashishta said once cursed it could not be changed but modified it so that all the seven Vasus woul immediately die after birth and return back to haven but Pirabaasan , the the one who took the cow, would live in the earth and get humiliated and suffered for what ever he did.

The Ashta vasus then went and requested river Ganga to be their mother. ganga agreed to their request.That was how Ganga became the queen of Shantanu. Now Ganga had to go back and hence she said the eighth vasu was "this boy whom I would take with me and hand over him when he was fit to join you", saying this she vanished.

Shantanu was grief stricken and came back to his palace.
After some years once Shantanu was strolling near river Ganga when he saw a very young boy was stopping the river flow with his arrows. He was surprised by the boy's archery and approached him and then Ganga appeared and told Shantanu that that boy was his own son Devavridhan  who had completed all his education and other martial arts and was ready to be left with him and leaving him with Shantanu she left. Thus Shantanu got united with his son.